Optical arrangements of the type mentioned at the beginning are known from practice and exist, in particular, in the form of microscopes. Such microscopes can be formed by confocal scanning microscopes. In scanning microscopy, a specimen or an object is scanned with the aid of an illuminating light beam. Use is freely made, for this purpose, of lasers as light source. The illuminating light from the laser is frequently transported to the microscope optics with the aid of glass fibres. The illuminating light generated by the laser is almost always linearly polarized, since usually at least one Brewster window is arranged in a laser resonator. In semiconductor lasers, the electron transition takes place only perpendicular to the p-n interface, and so linearly polarized light results here as well.
Particularly in confocal scanning microscopy, a specimen or an object is scanned in three dimensions with the aid of the focus of the illuminating light beam. A confocal scanning microscope generally comprises a light source, a focusing optics, with the aid of which the light from the light source is focused onto a pinhole stop—the so-called excitation stop—the beam splitter, a scanning device for beam control, a microscope optics, a detection stop and the detectors for identifying the detection light or fluorescent light. Illuminating light is mostly launched via a main beam splitter. The fluorescent light or reflected light coming from the specimen or the object passes via the same scanning device or the same scanning mirror back to the beam splitter or main beam splitter and passes this, thereupon being focused onto the detection stop downstream of which the detectors, mostly photomultipliers, are located. Detection light, which does not originate directly from the focus region, takes a different light path and does not pass the detection stop, and so point information is obtained which leads to a three-dimensional image through sequential scanning of the specimen or the object. A three-dimensional image is mostly obtained by layerwise acquisition of image data.
U.S. Pat. No. 5,161,053 discloses a confocal microscope in which light from an external light source is transported with the aid of a glass fibre to the beam path of the microscope, and the end of the glass fibre serves as point light source such that a mechanical stop is superfluous.
The reflectivity or transimissivity of beam splitters, and the reflectance of scanning mirrors is generally a function of polarization. Even high-quality beam splitters cannot be completely optimized with regard to independence of polarization, and so differences always occur in the reflection response and transmission response with reference to s-polarization and p-polarization. When the direction of linear polarization of the illuminating light beam fluctuates, something which frequently occurs particularly when light is launched with the aid of a glass fibre, this results in disturbing fluctuations in illuminating light at the specimen or the object, and thus also in fluctuations in the detection light. The fluctuations or rotations in the linear polarization in a monomode glass fibre are to be ascribed to mechanical movement of the fibre which cause stress-induced birefringence.
This problem can be eliminated with aid of polarization-maintaining fibres, a new set of problems arising to the effect that the linear polarization of the laser light during launching into the fibre must be correctly orientated relative to the preferred fibre axis. Relative movement between laser and fibre therefore also lead to changes in the transmission through the fibre, something which complicates the optical adjustment, inter alia.